1. The Field of the Invention
The present invention relates to a wide dynamic range transimpedance amplifier. More particularly, the present invention relates to a wide dynamic range transimpedance amplifier with automatic gain control and direct current (DC) cancellation control.
2. The Relevant Technology
Fiber optic networks use light signals to transmit data over a network. Although light signals are used to carry data, the light signals are typically converted into electrical signals in order to extract and use the data. The conversion of an optical signal into an electrical signal is often achieved utilizing a fiber optic receiver. A fiber optic receiver converts the optical signal received over the optical fiber into an electrical signal, amplifies the electrical signal, and converts the electrical signal into an electrical digital data stream.
The fiber optic receiver usually includes a photodiode that detects the light signal and converts the light signal into an electrical signal or current. A transimpedance amplifier amplifies the signal from the photodiode into a relatively large amplitude electrical signal. The amplified electrical signal is then converted into a digital data stream.
The optical signals that are converted into electrical signals by the fiber optic receiver, however, can vary significantly in both amplitude and power. The power of the optical signal is often related, for example, to the length of the optical fiber over which the optical signal was received, the laser source, etc. These and other factors result in optical signals whose incident power at the transimpedance amplifier can vary significantly.
Fiber optic receivers are only able to successfully receive and amplify optical signals that fall within a particular power range. In order for a fiber optic receiver to accommodate a wide range of optical signals, the fiber optic receiver and in particular, the transimpedance amplifier, should be able to detect and amplify very low levels of optical power as well as high levels of optical power. The range of signals that can be successfully amplified is therefore effectively limited by the incident optical power because the fiber optic receiver distorts or clamps signals whose optical power is too large and cannot recognize signals whose optical power is too low.
One problem with current transimpedance amplifiers is that extending the ability of the transimpedance amplifier to amplify signals with more optical power usually diminishes the ability of the transimpedance amplifier to amplify signals with low optical power. In other words, the maximum optical input power that can be accepted by the transimpedance amplifier while meeting signal integrity and bit error rate specifications is usually specified as the input optical overload. The minimum input power is specified as optical sensitivity. The transimpedance amplifier should be designed to maximize the optical overload and minimize the optical sensitivity. In most of the commercial or published transimpedance amplifiers, there is a direct tradeoff between the circuit optical (or current) sensitivity (or equivalent input current noise) and the optical (or current) overload. Some solutions to this problem, such as utilizing clamping circuitry or voltage regulators to assist in the amplification of optical signals with relatively large optical power, add both cost and complexity to the transimpedance amplifier of the fiber optical receiver. Without the aid of additional circuitry, the range of signals that can be successfully amplified is somewhat limited because the circuitry used for automatic gain control and DC cancellation introduces unwanted gain into the transimpedance amplifiers at large optical power.
The unwanted gain also has an adverse effect on the low frequency cutoff at higher optical powers. In other words, transimpedance amplifiers do not function at certain frequencies because the low frequency cutoff has been increased. The low frequency cutoff for these types of transimpedance amplifiers is related to the transconductance of the circuitry used for automatic gain control and DC cancellation. Thus, as the current of the input signal increases, the low frequency cutoff of the transimpedance amplifier is adversely affected.